In keeping with prior proposals, a basic self-clocking glyph code, such as shown in FIG. 1, typically is composed of an array 21 of elongated, slash-like symbols or "glyphs" 22 and 23 that are written with their longitudinal axes tilted at angles of approximately -45.degree. and +45.degree. with respect to the vertical axis 24 of a recording medium 25 to encode bit values of "1" and "0", respectively, or vice-versa, in each of the glyphs. These codes are "self-clocking" because they include an optically detectable symbol (a "glyph") for each of the values they encode. This means that the detection of the glyphs implicitly synchronizes the decoding process to the code once the decoding process has been properly spatially oriented with respect to the code. As will be evident, this implicit synchronization is valid so long as all of the glyphs are detected or otherwise accounted for in correct logical order. If, however, synchronization is lost, there is nothing to restore the implicit synchronization, so a loss of synchronization generally causes an implicitly synchronized decoding process to experience fatal error. For a more detailed discussion of these glyph codes and of techniques for decoding them, the following commonly assigned U.S. patent documents are hereby incorporated by reference: a copending application or Bloomberg et al. on "Self-Clocking Glyph Shape Codes," which was filed Aug. 18, 1992 under U.S. Ser. No. 07/931,554; Bloomberg U.S. Pat. No. 5,168,147 which issued Dec. 1, 1992 on "Binary Image Processing for Decoding Self-Clocking Glyph Shape Codes;" Bloomberg et al. U.S. Pat. No. 5,091,966, which issued Feb. 25, 1992 on "Adaptive Scaling for Decoding Spatially Periodic Self-Clocking Glyph Shape Codes;" and Stearns et al. U.S. Pat. No. 5,128,525, which issued Jul. 7, 1992 on "Convolution Filtering for Decoding Self-Clocking Glyph Shape Codes."
As will be seen, these prior proposals not only describe the use of two discriminable glyph shapes for the encoding of single bit binary values, but also teach that "glyph shape encoding" is extensible to the encoding of digital values of any given bit length, n, in each glyph by utilizing a code having 2.sup.n discriminable glyph shapes (where the discriminability between glyphs representing different values is based upon the distinctive rotational orientations and/or the distinctive geometric configurations of the different glyphs). Furthermore, another copending and commonly assigned U.S. patent application of David L. Hecht on "Self-Clocking Glyph Code Having Composite Glyphs for Distributively Encoding Multi-bit Digital Values," which was filed Dec. 12, 1991 under U.S. Ser. No. 07/814,842 and which also is hereby incorporated by reference, provides a technique for encoding a plurality of bit values in each of the glyphs by constructing the glyphs so that they each have a plurality of independently modulatable and readily distinguishable (e.g., substantially orthogonal) characteristics. Thus, even though a straightforward self-clocking glyph code of the type shown in FIG. 1 is featured in this disclosure to simplify the description, it will be evident that the broader aspects of this invention are applicable to other glyph codes, including more complex ones.
In practice, each of the glyphs 22 and 23 usually is defined by writing a recognizable pattern of "on" and "off" pixels into a two dimensional, array of pixel positions (i. e., a "symbol cell"). As a general rule, the pixels that form the body of the glyph (say, the "on" pixels) are written into contiguous pixel positions that are more or less centered within the symbol cell, and the other or "off" pixels are written into the remaining pixel positions of the symbol cell to provide a contrasting surround that demarks the edges of the glyph and separates it from its neighbors.
The symbol cells, in turn, ordinarily are tiled onto the recording medium in accordance with a preselected spatial formatting rule, so the logical order of the data values that the glyphs encode is preserved by the spatial order in which the glyphs are mapped onto the recording medium. For example, the symbol cells may be written on the recording medium in accordance with a regular and repeating spatial formatting rule that is selected to map the glyph encodings into a two dimensional, rectangular array of logical data blocks of predetermined size, such as data blocks having a 16 symbol cell.times.16 symbol cell format. These data blocks suitably are organized on the recording medium in left-to-right, top-to-bottom logical order.
The size of the symbol cells that are used for the glyphs depends on a variety of factors, including the spatial resolution of the printing process that is employed to write the glyphs, the type and extent of the degeneration that the printed glyph code is required to tolerate, and the spatial resolution of the lowest resolution scanning process that is expected to be able to recover the code. In view of these constraints, a 300 s.p.i. printer suitably centers the glyphs in 5 pixel.times.5 pixel or 7 pixel.times.7 pixel symbol cells. Even larger symbol cells can be employed for the glyphs if the increased granularity of the textured appearance of the printed glyph code is tolerable or unavoidable. As a general rule, of course, the smallest practical symbol cell size is esthetically most pleasing because the visual texturing appearance of the glyph code gradually blends as the symbol cell size is reduced. Indeed, codes composed of smaller symbol cells tend to have generally uniform gray scale appearances when they are viewed under ordinary lighting at normal viewing distances.
The existing techniques for decoding self clocking glyph codes are designed to be initialized at or near the center of a reference glyph that occupies a known spatial position relative to the remainder of the glyph code (for example, the glyph in the upper, lefthand corner of a rectangular array of glyphs). Thus, accurately locating this reference glyph clearly is a key to spatially synchronizing such a decoding process with the glyph code.
While synchronous initialization of the decoding process is a necessary condition for orderly decoding of a self-clocking glyph code, it may not be a sufficient condition to ensure accurate decoding of the code because the scanned-in image of the glyph code pattern often is distorted by skew and/or scaling errors. For this reason, prior decoding processes generally have attempted to determine the relative spatial positions of three or more reference glyphs in recognizeable positions that have a known nominal, non-colinear relationship with respect to each other (such as the corner glyphs of a rectangular array of glyphs) with sufficient precision to compute skew and scaling correction factors. These correction factors then are used to adjust the angle and magnitude of a vector that dictates the direction and magnitude of the jumps the decoding process makes while it is advancing from glyph-to-glyph.
As will be appreciated, anything that prevents the decoding process from establishing and maintaining proper spatial synchronization with the glyph code threatens to defeat the decoding. Indeed, as a general rule, any glyphs that are isolated from synchronously readable glyphs may be unsynchronizable, especially in the interior of a glyph code pattern. Spatial synchronization is, of course, required to enable the decoding process to preserve the logical ordering of the data that is encoded in the glyphs. Minor synchronization errors affecting a limited number of glyphs may be tolerable if the code has sufficient error correcting capacity (i. e., error correction code protection) to correct for the resulting decode errors, but more extended synchronization errors generally result in incomplete reading of the encoded information. Ultimately, this performance degradation can become sufficiently severe to be unacceptable.
Accordingly, it will be evident that one of the more significant disadvantages of spatially synchronizing a glyph decoding process through the use of a global imaginal reference that is computed from the spatial locations of certain key glyphs, such as the corner glyphs of a rectangular glyph pattern, is that various factors may prevent the spatial location of one or more of those key glyphs from being determined with sufficient precision to compute a valid reference, thereby preventing the decoding process from achieving and/or maintaining spatial synchronism with the glyph code. Moreover, even when a valid global reference can be computed, there is an appreciable risk that the decoding process will lose synchronization. For example, the scanned-in image of the glyph code might be sufficiently degraded to cause the decoding process to erroneously branch from one row of glyphs to an adjacent row, or to skip over a glyph without accounting for it, or to miscount the number of unreadable glyphs in a damaged region of the image. Unfortunately, none of the known self-clocking glyph codes or related decoding processes enable the decoding process to re-establish proper synchronization once it has been lost, so a loss of synchronization usually causes these existing decoding processes to degenerate into an irreversible failure mode.
The problems that have been encountered with establishing and maintaining proper spatial synchronization during the decoding of self-clocking glyph codes have been compounded by the lack of reliable techniques for detecting asynchronous operation of the decoder. Glyph centers have been counted during the decoding of these codes on the theory that a miscount over any predefined segment of the formatted code will signal a loss of synchronization. This is a weak synchronization validation process if part of the glyph code pattern (e.g., a portion of its periphery) is missing or otherwise undetectable. there may be an ambiguity with respect to the number of glyphs that are present in the image and with respect to what part of the glyph pattern is present in the image. Thus, the decoding process may provide invalid results, without providing any warning that the results are invalid.
Image rotation of 90.degree., 180.degree., 270.degree. and reflection result in rotational and mirror image transformations of the glyph code pattern, and these transformations can alter the apparent spatial order of the glyphs and/or the apparent rotational orientation of the individual glyphs. Indeed, this is an especially significant issue when the glyphs encode the data values in their rotational orientations, such as when the glyphs are slash-like symbols that are oriented at .+-.45.degree. with respect to the longitudinal axis of the recording medium to encode high ("1") and low ("0") logic level bits, respectively, because such glyphs often are written in patterns that have rotational symmetry. Unfortunately, the orientation of scanned-in glyph code patterns frequently involves some ambiguity. For example, there typically is a four-fold rotational ambiguity in the case of a glyph code that is composed of a square array of slash-like glyphs of the above-described type that are written into square symbol cells, and a two-fold ambiguity in the case of a glyph code that is composed of a rectangular array of such glyphs.
Some of the commercially available two dimensional data codes, such as Vericode.TM. by Veritex and Datacode.TM. by Data Code International, have distinctive reference marks along one or more of their borders that can be used for scaling and for explicitly spatially synchronizing the decoding of those codes However, these reference marks are visually distinctive so they tend to increase the visual obtrusiveness of those codes. One of the principal advantages of self-clocking glyph codes is that they tend to be esthetically pleasing because of their non-obtrusive visual appearance, so it would be counterproductive to supplement them with reference patterns that have such a strong visual impact.